Enthalpy is the exchange of thermal energy during chemical reactions, measured in kJ/mol. Exothermic reactions release heat, endothermic reactions absorb heat. Energy level diagrams and calorimeters are used to measure enthalpy change. Calculation requires knowledge of water’s specific heat capacity, mass, and temperature change.
Enthalpy is the exchange of thermal energy that occurs during chemical reactions. It has the symbol H and is measured in kJ/mol, or kilojoules per mole. The energy exchanged with the surrounding environment at constant pressure is called the enthalpy change of a reaction. To measure the variation, standard conditions are used, including a pressure of 1 atmosphere and a temperature of 298 Kelvin (77°F or 25°C).
When the reaction gives off heat, it is said to be exothermic. In this situation, the enthalpy change is negative, as the reaction is moving from high energy to low energy due to the loss of thermal energy to the surroundings. If energy flows from its surroundings into a system, or heat is absorbed, it is said to be endothermic. In this case, the change is positive as the system is gaining energy in the form of heat.
Energy level diagrams are the most common way to show such heat changes. Two general diagrams are used, one for exothermic reactions and one for endothermic reactions. For an exothermic reaction, the energy of the reactants of the reaction would be greater than the energy of the resulting products. The opposite is true for endothermic reactions. The change is clearly marked on both diagrams.
Two ways to measure an enthalpy change are with a simple calorimeter or a simple flame calorimeter. A simple calorimeter consists of a container of water, with the heat transferred to the water. The temperature variation of the water that is recorded will be due to the energy exchange of the reaction. When using a simple calorimeter, the reaction takes place within the calorimeter’s water.
With a simple flame calorimeter the reaction takes place outside the device. During the reaction, heat must be transferred to the water inside the calorimeter. This type of calorimeter is used to measure the variation of combustion reactions.
To calculate the enthalpy change, you need to know three things. The first is a constant, which is the specific heat capacity of water, which is 4.2 J/g/°C. It is necessary to measure the mass of the water and the extent of the change in temperature of the water. The formula for calculating this change is 4.2 x water mass x temperature change.
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